Membrane assembly for supporting a biofilm

ABSTRACT

A cord for supporting a biofilm has a plurality of yarns. At least one of the yarns comprises a plurality of hollow fiber gas transfer membranes. At least one of the yarns extends along the length of the cord generally in the shape of a spiral. Optionally, one or more of the yarns may comprise one or more reinforcing filaments. In some examples, a reinforcing yarn is wrapped around a core. A module may be made by potting a plurality of the cords in at least one header. A reactor may be made and operated by placing the module in a tank fed with water to be treated and supplying a gas to the module. In use, a biofilm covers the cords to form a membrane biofilm assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Application Serial No.16/998,704, filed Aug. 20, 2020, which is a of continuation of U.S.Application Serial No. 14/769,461, filed Aug. 21, 2015, issued on Sep.22, 2020 as US Pat. No. 10,781,119, which is a National Stage Entry ofInternational Application No. PCT/US2013/027435, filed Feb. 22, 2013.U.S. Application Serial Nos. 16/998,704 and 14/769,461 and InternationalApplication No. PCT/US2013/027435 are incorporated herein by reference.

FIELD

This specification relates to wastewater treatment, to membrane biofilmreactors, and to assemblies of gas permeable membranes for supporting abiofilm.

BACKGROUND

In a membrane biofilm reactor (MBfR), a membrane is used to both supporta biofilm and to transport a gas to the biofilm. Membrane biofilmreactors were recently reviewed by Martin and Nerenberg in “The membranebiofilm reactor (MBfR) for water and wastewater treatment: Principles,applications, and recent developments” (Bioresour. Technol. 2012).Membrane-aerated biofilm reactors (MABR) are a subset of MBfRs in whichan oxygen containing gas is used. MABRs were reviewed by Syron and Caseyin “Membrane-Aerated Biofilms for High Rate Biotreatment: PerformanceAppraisal, Engineering Principles, Scale-up, and DevelopmentRequirements” (Environmental Science and Technology, 42(6): 1833-1844,2008).

U.S. Pat. No. 7,169,295 describes a membrane supported biofilm reactorwith modules having fine hollow fiber membranes. The membranes are madefrom dense wall polymethyl pentene (PMP) used in tows or formed into afabric. The membranes are potted in a header of a module to enableoxygen containing gas to be supplied to the lumens of the hollow fibers.The reactor may be used to treat wastewater. Mechanical, chemical andbiological methods are used to control the thickness of the biofilm.

SUMMARY OF THE INVENTION

This specification describes an assembly, alternatively called a cord,which may be used for supporting a biofilm. The cord comprises aplurality of hollow fiber gas transfer membranes. The cord mayoptionally also comprise one or more reinforcing filaments.

The cord may comprise a plurality of yarns. At least one of the yarnscomprises a plurality of gas transfer membranes. At least one of theyarns extends along the length of the cord generally in the shape of aspiral. In some cases, the cord has a core and one or more wrap yarns.In some other cases, the cord comprises a set of braided yarns.

The cord preferably has an outside diameter in the range of about 0.3 mmto 2.0 mm. The gas transfer membranes preferably have an outsidediameter that is less than 200 microns. The sum of the circumferences ofthe gas transfer membranes is preferably at least 1.5 times thecircumference of the smallest circle that can surround the cord. In use,a biofilm covers the cord and the outer surface of the biofilm issubstantially round.

A module may be made by potting a plurality of cords in at least oneheader. The cords are generally independent of each other except in theheader. A reactor may be made by placing the module in a tank adapted tohold water to be treated and providing a gas delivery system. A processfor treating wastewater comprises steps of feeding water to the tank andsupplying a gas to the module. In use, a biofilm may cover a cord toform a membrane biofilm assembly.

The cord, module, reactor and process may be used to treat water, forexample in, or in the manner of, an MBfR.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a photograph of a cord.

FIGS. 2 to 4 are photographs of alternative cords.

FIG. 5 is a schematic drawing of a machine for making a cord.

FIGS. 6 to 8 are schematic drawings of steps in a process for making amodule comprising a plurality of cords.

FIG. 9 is a schematic drawing of a module comprising a plurality ofcords.

FIG. 10 is a schematic drawing of a reactor comprising the module ofFIG. 9 .

FIGS. 11 to 14 are drawings of the cords in FIGS. 1 to 4 respectively.

DETAILED DESCRIPTION

FIGS. 1 to 4 and FIGS. 11 to 14 each show a cord 10 comprising aplurality of yarns 8. At least one of the yarns 8 comprises a pluralityof hollow fiber gas transfer membranes 14. Preferably, at least one ofthe yarns 11 comprises at least one reinforcing filament 34.

At least one of the yarns 8 extends along the length of the cord 10generally in the shape of a spiral and may be referred to as a spiralyarn. Preferably, at least one spiral yarn 8 is wrapped at leastpartially around the outside of the other yarn or yarns 8 of the cord10.

In the cords 10 of FIGS. 1 to 4 and FIGS. 11 to 14 , the yarns 8 arearranged to provide a core 12 and one or more wraps 18. A wrap 18travels in a spiral that is always outside of another yarn 8. Forexample, a wrap 18 may spiral around a core 12, or around one or moreother wraps 18 that spiral around the core 12. An only or outer wrap 18is located entirely outside of the other yarn or yarns 8 of a cord 10.Alternatively, a spiral yarn 8 may be wrapped only partially outside ofthe other yarns or yarns 8 or a cord. For example, two or more spiralyarns 8 may be twisted around each other or four or more spiral yarns 8may be braided together to form a cord 10 with no wrap 18. In anotheralternative, four or more spiral yarns 8 may be braided together arounda core 12.

Each cord 10 has a plurality of hollow fiber gas transfer membranes 14.The gas transfer membranes 14 may be located in a core 12, in one ormore wraps 18, or in another yarn 8. The gas transfer membranes 14 arepreferably provided in the form of a multi-filament yarn having aplurality of gas transfer membranes which may be called a gas transfermembrane yarn 15. Optionally, a cord may also have a reinforcing yarn16. The reinforcing yarn has one or more reinforcing filaments 34. Ayarn 8 having both a plurality of gas transfer membranes 14 and at leastone reinforcing filament 34 may be called both a gas transfer membraneyarn 15 and a reinforcing yarn 16.

The outside diameter of a cord 10 is preferably in a range of about 0.3to 2.0 mm. The outside diameter of the cord 10 may be measured as thelargest width of a cord measured through its longitudinal axis or as thediameter of the smallest hole that the cord 10 will pass through.Anomalies, defects or non-repeating bumps are ignored in thesemeasurements.

Generally speaking, a core 12 provides mechanical strength, defines alongitudinal axis of the cord 10, supports any wraps 18, and may alsocomprise gas transfer membranes 14. A wrap 18, or other yarn outside ofthe core 12, may do one or more of: protect the core 12 or anotherunderlying yarn 8, comprise gas transfer membranes 14, or contribute tothe mechanical strength of the cord 10.

A core 12 can be made of one or more monofilament yarns ormulti-filament yarns. A Multi-filament yarn may comprise filaments thatare braided, twisted or otherwise united, or filaments that are merelycollected together in a bundle or tow. Multiple yarns may be arranged asparallel warp yarns or twisted, braided or otherwise united. For examplea core 12 may consist essentially of a single monofilament yarn; asingle multi-filament yarn; or, an assembly of about 2 to 6 monofilamentor multi-filament yarns arranged in parallel or twisted or braidedtogether. An assembly of twisted or braided yarns may be preferred sincethe assembly will be more flexible than a single monofilament of thesame outer diameter. A core 12 typically, but not necessarily, comprisesat least one reinforcing yarn 16. A core 12 may optionally comprise oneor more gas transfer membrane yarns 15.

A wrap 18 is typically a multi-filament yarn. A multi-filament yarn maycomprise filaments that are twisted or otherwise united or filamentsthat are merely collected together in a bundle or tow. A wrap 18 can bewrapped around a core 12 in a clockwise spiral or a counterclockwisespiral. Alternatively, a cord 10 may have at least one wrap 18 in eachdirection or no wrap 18. A wrap 18 can be a gas transfer membrane yarn15, a reinforcing yarn 16, or both.

A reinforcing filament 34 can be made of any water-resistant andnon-biodegradable polymer such as polyethylene, nylon or polyester,preferably nylon or polyester. A reinforcing filament 34 is typicallysolid. Gas transfer membranes 14 tend to be expensive and weak relativeto reinforcing filaments 34 made of common textile polymers such asnylon or polyester. A reinforcing yarn 16 can be a monofilament ormulti-filament yarn. In the case of a multi-filament yarn, thereinforcing filaments 34 may be braided, twisted or otherwise united, orfilaments that are merely collected together in a bundle or tow.Optionally, a yarn 8 may comprise one or more reinforcing filaments 34mixed with the gas transfer membranes 14.

The gas transfer membranes 14 preferably have an outside diameter of 500microns or less, more preferably 200 microns or less, optionally 100microns or less. The hollow area of a hollow fiber (meaning the crosssectional area of the lumen of a fiber as a percentage of its totalcross sectional area) is preferably at least 20%, for example in therange of 20-50%. For example, a gas transfer membrane 14 may have anoutside diameter in the range of about 30-70 microns and an insidediameter of about 15-50 microns. The wall thickness of a gas transfermembrane 14 may be 20 microns or less.

The gas transfer membranes 14 are preferably handled in a multi-filamentgas transfer membrane yarn 15. A gas transfer membrane yarn 15 may havebetween 2 and 200, between 12 and 96, or between 10 and 60 individualfilaments of gas transfer membrane 14. A gas transfer membrane yarn 15used as a wrap 18 is preferably not tightly twisted, braided or crimpedto allow the individual gas transfer membranes 14 to spread out over anunderlying yarn 8. A gas transfer membrane yarn 15 may be made byre-winding gas transfer membranes 14 from multiple take up spools incombination onto another spool.

A gas transfer membrane yarn 15 may be provided, in a core 12, either asa central yarn or as a warp parallel to a central reinforcing yarn 16;in one or more wraps 18; or, in another spiral yarn 8. It is desirablefor gas transfer efficiency to have the gas transfer membranes 14 nearthe outer surface of the cord 10. However, the gas transfer membranes 14are typically fragile and they are more likely can be damaged if theyform the outer surface of a cord 10. Accordingly, it is preferable for areinforcing yarn 16 to be used as an outer wrap 18. In this case, a gastransfer membrane yarn 15 may be used in the core 12 or in an inner wrap18.

The gas transfer membranes 14 may be porous, non-porous or semi-porous.Composite membranes, for example having a non-porous membrane layer, anda semi-porous or porous support layer, may also be used. Asymmetricmembranes, for example having a non-porous region and an integralsemi-porous or porous region, may also be used.

Porous gas transfer membranes 14 may have pores up to themicrofiltration range. Wetting is avoided by choosing hydrophobicmaterials or treating the hollow fibers 14 to make them hydrophobic.Porous hollow fibers 14 may be made, for example, using polyethylene,polyvinylchloride, polypropylene or polysulfone.

Non-porous gas transfer membranes 14, including dense wall gas transfermembranes 14, may be made from a thermoplastic polymer, for example apolyolefin such as polymethyl pentene (Poly (4-methylpentene-1) or PMP),polyethylene (PE) or polypropylene (PP). PMP is sold, for example, byMitsui Petrochemical under the trade mark TPX. The polymer may be meltspun into a hollow fiber. The gas transfer membranes 14 may be callednon-porous if water does not flow through the fiber walls by bulk oradvective flow of liquid even though there are small openings throughthe wall, typically in the range of 4 or 5 s in the case of melt spunPMP. However, oxygen or other gases may permeate or travel through thefiber walls. In a dense walled hollow fiber 14, gas travel is primarilyby molecular diffusion or dissolution-diffusion which occurs whenopenings in the fiber walls are generally less than 30 s. Gas transfermembranes 14 as described in U.S. Pat. 7,169,295, which is incorporatedby reference, may be used.

The term porous has been used to refer to any structure having openingslarger than in a dense wall, for example having openings of 30 or 40 sor more, but without openings large enough to be wetted or transportliquid water by advective, Poiseuille or bulk flow. In thisspecification, membranes with openings in this size range are referredto as semi-porous.

Gas transfer membranes 14 may alternatively be made by mechanically orthermally treating a melt spun thermoplastic polymer after spinning toincrease its permeability to oxygen without making the fiber wettable orcapable of permitting advective flow of liquid water. Spinning orpost-treatment steps that can be used or controlled to increasepermeability include the spinning speed or drawing ratio, the quenchingconditions such as temperature or air flow rate, post annealing, if any,stretching and heat setting. The resulting fibers may have a denselayer, with openings ranging from the size of openings in the rawpolymer to 30 or 40 s, on either the inside of the fiber, the outside ofthe fiber or both, with the remaining parts of the fiber being porous orsemi-porous. For example, U.S. Pat. No. 4,664,681, issued on May 12,1997, to Anazawa et al. describes, in examples 4 and 6, processes formelt-spinning and post-processing PE and PP to produce acceptable fibreswhile other fibers are made from PMP or polyoxymethylene. Processesdescribed in “Melt-spun Asymmetric Poly (4-methyl-1-pentene) HollowFibre Membranes”, Journal of Membrane Science, 137 (1997) 55-61,Twarowska-Shmidt et al., also produce acceptable fibres of PMP and maybe adopted to produce fibres of other polyolefins such as PE or PP. Inone example, the mean pore size of the fibers produced is just over 40s. In U.S. Pat. No. 4,664,681, membranes are melt spun, stretched (byproducing the membrane at a high draft ratio) under weak cooling andthen heat treated. The resulting membranes are asymmetric containing adense layer with substantially no pore with a diameter of 30 s or moreand a microporous layer having larger pores. The non-porous layer is theouter surface of the fiber and so the fiber is non-wetting.

Another alternative process for making gas transfer membranes 14 is tomake an asymmetric outside dense skin membrane with a spongysubstructure by the non-solvent induced phase separation (NIPS) process.Polymers typically used for this process are polysulfone, celluloseacetate and polyimide. Other alternative methods of making gas transfermembranes 14 may include, for example, meltblown extrusion, flashspinning, and electrospinning.

In general, silicon rubber or PDMS have very high oxygen permeabilitybut cannot be processed using many textile techniques and are notavailable in small diameter fibers. PMP has higher oxygen permeabilitythan PE or PP, but it is more expensive. Porous membranes have highoxygen permeability but they are prone to wetting in use. Accordingly,dense wall polymeric gas transfer membranes 14, for example of PE. PP orPMP, with a wall thickness of 50 microns or less, preferably 20 micronsor less, or polymeric membranes that are not entirely dense walled buthave a nonporous or semi-porous layer, are preferred but not essential.

A yarn 8 in a core 12, whether it is a gas transfer yarn 15 or areinforcing yarn 16, may be called a warp 26. A wrap 18 is preferablyapplied around a core 12 or another underlying yarn 8 with some tensionto cause its filaments to spread on the surface of the core 12. Wrappingmay be done with a pitch ratio (pitch divided by the diameter of thecore) of between 1 and 5. Wrapping can be in one direction only, but ispreferably done in both directions. There may be 1 or more, for examplebetween 1 and 3, wraps 18 in one direction. Gas transfer membranes 14are preferably directly exposed over at least 25% of the surface of thecore 12 although oxygen can also travel from a gas transfer membrane 14through an overlying yarn 8.

A cord 10 may consist of only a set of twisted yarns 8. However, merelytwisted yarns 8 may tend to partially untwist and separate in use.Accordingly, it is preferable to use twisted yarns 8 as a core 12 withat least one wrap 18 wrapped around the core 12 in the directionopposite to the twist of the core 12. A braided core 12 is more stable.Spiral yarns 8 may be added as a braid around a core 12, but a wrap 18can be made at a faster line speed with a less complicated machine. Acord structure using one or more wraps 18 also allows for a reinforcingyarn 16 to be used as an outer wrap 18 to help protect an interior gastransfer membrane yarn 15.

In the examples of FIGS. 1 to 4 and FIGS. 11 to 14 , gas transfermembrane yarns 15 are made up of 48 dense wall PMP hollow fiberfilaments. The filaments have an outside diameter of less than about 70microns and a wall thickness of less than 20 microns. Reinforcing yarns16 are either monofilament yarns or multi-filament yarns of polyester(PET).

In FIG. 1 and FIG. 11 , a first cord 10 a has a core 12 made up of asingle monofilament reinforcing yarn 16 and a two multi-filament gastransfer membrane yarns 15 applied as warps 26 parallel to thereinforcing yarn 16. The core 12 is covered with two wraps 18, one ineach direction, with a wrapping pitch of 1.8. Each wrap 18 is a singlemulti-filament gas transfer membrane yarn 15.

In FIG. 2 and FIG. 12 , a second cord 10 b has a core 12 comprising areinforcing yarn 16 and a gas transfer membrane yarn 15. Each of theseyarns in the core 12 is an untwisted multi-filament yarn, alternativelycalled a tow. The second cord 10 b also has two wraps 18, one in eachdirection. Each wrap 18 is an untwisted multi-filament reinforcing yarn16.

In FIG. 3 and FIG. 13 , a third cord 10 c has a core 12 comprising a setof multifilament reinforcing yarns 16 braided together. The third cord10 c also has an inner wrap 18 comprising an untwisted multifilament gastransfer membrane yarn 15 and an outer wrap comprising an untwistedmultifilament reinforcing yarn 16.

In FIG. 4 and FIG. 14 , a fourth cord 10 c has a core 12 comprising aset of multifilament reinforcing yarns 16 braided together and anuntwisted multifilament gas transfer membrane yarn 15 as a warp 26parallel to the reinforcing yarns 16. The gas transfer membrane yarn 15is parallel with the core 12 but not braided with the reinforcing yarns16. The third cord 10 c also has a wrap 18 comprising an untwistedmultifilament reinforcing yarn 16.

FIG. 5 shows a machine 20 for making a cord 10. The machine 20 is builton frame 22 that supports the different components and aligns them. Oneor more warps 26 are supplied to the machine 20 from a creel 24. Thecreel 24 has stationary bobbin holders, guides and tensioning devices,as found in other textile equipment. The warps 26 pass through adistributor 28. The distributor 28 may have a central opening and one ormore eyelets around the central opening. A warp 26 is unwound from abobbin on the creel 24, positioned to the top of the distributor 28through a roller and fed vertically down through the distributor 28. Atake up winder (not shown) pulls the cord 10 downwards through themachine 20 and onto a bobbin. The one or more warps 26 form the core 12of the cord 10.

One or more spindles 30, or other yarn wrapping devices, are locatedbelow the distributor 28. Each spindle 30 is loaded with a yarn andwraps the yarn around the one or more warps 26 of the core 12 as theypass through the spindle 30. Due to the downward movement of the core12, each wrapped yarn forms a spiral wrap 18. The machine 20 may alsohave alignment guides (not shown) to keep the core 12 aligned with thecentral axis of the spindles 30 and to reduce vibration of the core 12.

An example of a suitable spindle 30 is a Temco® spindle model MSE150 byOerlikon Textile. Each spindle 30 has an electrical motor and a hollowcore and holds a bobbin of wrap yarn. The spindle 30 is positioned sothat its central axis coincides with the core 12. In the machine 20 ofFIG. 2 , there are two spindles 30, one rotating clockwise and the otherrotating counter clockwise. The spindles 30 can rotate at an adjustablespeed of up to 25,000 rpm to provide a controllable pitch.Alternatively, a rotating creel may be used in place of the spindle 30.In a rotating creel, bobbins are mounted on a wheel that rotates in onedirection around the core 12 without being in contact with it. Eachbobbin is preferably equipped with tension control.

A plurality of cords 10, for example 100 or more, may be made into amodule generally in the manner of making an immersed hollow fibermembrane filtration module. At least one end of each of the cords 10 ispotted in a block of a potting material such as thermoplastic orthermosetting resin which is sealed to a pan to form a header. The endsof the gas transfer membranes 14 are made open to the inside of theheader, for example by cutting them open after potting. The other endsof the cords 10 may be potted in another header with the ends of the gastransfer membranes 14 open or closed, closed individually, or loopedback and potted in the first header. A port in the header allows a gasto be fed to the lumens of the gas transfer membranes 14. The gas may befed to the gas transfer membranes 14 in a dead end manner or withexhaust through a second header.

As one example, the composite fibers 10 may be assembled into modulesand cassettes according to the configuration of ZeeWeed 500® immersedmembrane filtration units sold by GE Water & Process Technologies.Sheets of cords 10 are prepared with the composite fibers 10 generallyevenly spaced in the sheet. Multiple sheets are stacked on top of eachother to form a bundle with adjacent sheets spaced apart from eachother. The bundle is potted. After the potting material cures, it is cutto expose the open ends of the gas transfer membranes and sealed to aheader pan. Several such modules may be attached to a common frame withtheir ports manifolded together to form a cassette. Various usefultechniques that may be used or adapted for making a module are describedin U.S. Pat. 7,169,295, U.S. Pat. 7,300,571, 7303676, U.S. Publication2003/01737006 A1 and International Publication Number WO 02/094421, allof which are incorporated by reference. Alternatively, other knowntechniques for making a hollow fiber membrane module may be used.

Referring to FIG. 6 , multiple cords 10, or an undulating cord 10, arelaid out on a flat jig or drum to provide a set of generally parallelsegments of cord 10 in a sheet 38. The segments of cord 10 may be keptevenly spaced from each other in the sheet 38, for example by a wovenfilament 40 or a strip of hot melt adhesive 42. When segments of cord 10are used, the ends of the cords 10 may be sealed, for example by meltingthem with an iron or heated cutter along a sealing line 44. Multiplesheets 38 may be stacked on top of each other, preferably with the endsof adjacent sheets 38 separated by spacers. The end of the set of sheets38 is dipped in a potting mold 46 filled with a potting resin 48. Thepotting resin 48 may be, for example, a polyurethane resin formulated topenetrate into the yarns 8 to seal around the various filaments of thecord 10.

Referring to FIG. 7 , the set of sheets 38 is removed from the pottingmold 46 after the potting resin 48 is cured. To expose open ends of thegas transfer membranes 14, the potting resin 48 is cut through alongcutting line 50. The other end of the set of sheets 38 may be potted inthe same manner. Optionally, one or both of the blocks of potting resin48 may be cut to expose open ends of the gas transfer membranes 14.

Referring to FIG. 8 , a header 60 is formed by sealing the block ofpotting resin 48 to a header pan 52. The header pan 52 may be made ofmolded plastic and has an outlet 54. The block of potting resin 48 maybe held in the header pan 52 by an adhesive or a gasket 56 between theperimeter of the potting resin 48 and the header pan 52. Optionally, asecond potting material 58 may be pored over the potting resin 48. Thesecond potting material may further seal the cords 10 or the pottingresin 48 to the header pan 52, or may cushion the cords 10 where theyexit from the header 60. A similar header 60 may be made at the otherend of the set of sheets 38.

Referring to FIG. 9 , a module 66 has two headers 60 with cords 10extending between them. The headers 60 are preferably vertically alignedand held apart by a frame 62. The length of the cords 10 betweenopposing faces of the headers 60 may be slightly greater than thedistance between the opposed faces of the headers 60. In this case, thecords 10 have some slack and can sway. The cords 10 are preferably notconnected to each other between the headers 60. Although one cord 10 maycontact another as it sways, the movement of a cord 10 is generallyindependent of other cords 10. Multiple modules 60 may be held in acommon frame 62. The frame 62 may also hold an aerator 68 near thebottom of a module 66.

When used for wastewater treatment, the cords 10 are immersed in abioreactor and a gas, for example air, oxygen, hydrogen or another gas,is fed through the lumen of the gas transfer membranes 14. A biofilmdevelops on the outside surface of the cords 10, and anchors itself byfilling the gaps between filaments. The resulting membrane biofilmassembly has a generally circular cross section. The cross section ofthe membrane biofilm assembly has a diameter of about 0.5 to 3 mmdetermined assuming that the biofilm forms a film extending no more than0.5 mm beyond the outer diameter of the core 12. A more typical biofilmthickness is in the range of 0.05 to 0.2 mm. The sum of thecircumferences of the gas transfer membranes 14 multiplied by the lengthof the cord approximates the active gas transfer surface area while thecircumference of the outside of the biofilm multiplied by the length ofthe cord 10 gives the biofilm area. The sum of the circumferences of thegas transfer membranes 14 is preferably at least 1.5 times thecircumference of a circle having the outside diameter of the cord 10.The sum of the circumferences of the gas transfer membranes 14 is alsopreferably at least 1.5 times the circumference of the attached biofilmwhen in use.

Modules of the cords 10 may be deployed in a membrane biofilm reactor(MBfR) by immersing them in an open tank in manner similar to the use ofZeeWeed 500 immersed hollow fibre filtering membranes. Although thecords 10 will be used to support and transport gas to a biofilm and notfor filtration, various system design and operating features of theZeeWeed 500 system can be adapted. As mentioned above, the cords 10 maybe potted with an orderly spacing between them. The module configurationwith two headers may be used but modified to use one header of eachelement for introducing the fresh gas and one header for ventingexhausted gas. ZeeWeed cassette frames may be used to facilitatedeploying multiple modules with the cords 10 oriented vertically intoopen tanks. Gas sparging by way of bubbles produced below or near thebottom of the modules can be provided at a low rate to renew the liquidaround the cords 10. Gas sparging at a higher rate may be used to helpcontrol biofilm thickness either by the direct action of bubbles, bubblewakes or bubble pressure effects on the biofilm, or by causing cords 10mounted with slack between the headers to sway in the water to produceturbulence or contact between cords 10. Optionally, gas exhausted fromthe cords may be recycled for use in gas sparging.

Referring to FIG. 10 , a module 66 is immersed in a tank 70. The tank 70is filled with water to be treated from an inlet 72. Treated water isremoved through an outlet 74. Optionally, water may recirculate from theoutlet 74 to the inlet 72 to provide a flow of water through the module66, mix the tank 70, or to maintain desired conditions in the tank 70.Air, or another gas, is blown into, or drawn out of, the module 66 by aprocess gas blower 76. In the example shown, the gas is blown into oneheader 60, travels through the cords 10, and exhausted from the otherheader 60. A throttle valve 78 may be used to increase the gas pressurein the cords 10. A sparging gas blower 80 blows air or recycled exhaustgas from the module 66, or both, to the aerator 68 when required formixing the tank 70 or controlling the thickness of the biofilm on thecords 10.

Optionally, the aerator 68 may comprise a supply pipe 82 and atransducer 84. The transducer 84 collects gas ejected from the supplypipe in a pocket below a shell 86. The pocket of gas grows larger as gasis accumulated as shown in the first two compartments of the shell 86,counting from the left side of the shell 86. When the pocket of gasextends to the bottom of J shaped tube, as in the third compartment ofthe shell 86, the gas is released through the J shaped tube as shown inthe last compartment of the shell 86. In this way, large bursts ofbubbles are released periodically without requiring a large volume ofgas to be continuously pumped into the tank 70. Excessive scouring gasconsumes energy and may disturb desirable anoxic or anaerobic conditionsin the tank 70. Periodic large bursts of bubbles can be more effectivefor renewing the water around the cords 10 or removing biofilm from thecords 10 than the same amount of gas supplied as a continuous stream ofbubbles.

In some prior MBfRs, silicon rubber or polydimethylsiloxane (PDMS) arecoated over a flat substrate to make a flat sheet membrane. Whilesilicon and PDMS are highly permeable to oxygen, such a flat sheetmembrane can provide a surface area for oxygen transfer to surface areaof biofilm ratio of only about 1. Further, with reasonably large sheetsit is difficult to renew water to be treated along the edges of thesheet or remove excess biofilm from the edges of the sheet. Accordingly,the sheets are often separation by a substantial spacing and the totalbiofilm area in a tank may be low.

In U.S. Pat. No. 7,169,295 a membrane supported biofilm reactor hasmodules made with fine hollow fiber membranes. The fine hollow fibershave a thin wall which allows for good gas transfer efficiency even whenheat spun polymers are used. However, the fine hollow fibers are alsoeasily damaged. A tow module described in US 7,169,295, although usefulin some applications, has loose and exposed hollow fibers which areprone to damage and to being clumped together by biofilm in otherapplications. Sheet modules described in US 7,169,295 are more resilientand can provide a surface area for oxygen transfer to surface area ofbiofilm ratio of more than 1. However, like silicon flat sheet module,these sheet modules are still subject to total biofilm area limitations.

The cords 10 described above provide a useful alternative gas transfermodule configuration. The use of fine hollow fiber gas transfermembranes 14 allows for good gas transfer efficiencies even when usingmelt spun polymers and a surface area for oxygen transfer to surfacearea of biofilm ratio of more than 1. The fine hollow fiber membranesare not loose and exposed. Yet since the cords 10 can move generallyindependently and do not form a solid sheet, fresh liquid and bubblesused to scour the biofilm to control its thickness can reach cords 10located in the interior of a module. Movement of the cords 10, orcontact between cords 10, may also help control biofilm thickness.Further, the total biofilm surface area can be increased relative to asheet form module.

In a calculated example, a cord 10 comprises a core 12 and two wraps 18,one in each direction, of gas transfer membrane yarns 15. The outsidediameter of the cord is 1 mm. The wrapping pitch is 5 core diametersresulting in a wrap 18 helix length of 1.18 times the cord length. Thecord 10 has 57 meters of 70 micron outside diameter PMP gas transfermembrane 14 per meter of cord 10 length. The surface area of the gastransfer membranes per unit length of cord is 3.04 times the outersurface area of the cord, calculated based on the circumference of a 1mm circle.

A biofilm on the cord 10 is assumed to have a thickness of 0.2 mm givinga membrane biofilm assembly diameter of 1.4. The cords 10 are potted ina module in a rectilinear grid with a 0.7 mm gap between their outsidesurfaces. With biofilm attached, the gap between adjacent cords in aline is 0.4 mm. Using ZeeWeed module moldings, a module 66 has 16 rowsof 340 cords 10 each, or 5440 cords 10. The exposed length of each cordis 1.9 m giving a biofilm area per module of 45.5 square meters. Using aZeeWeed frame, a cassette with a 3.7 square meter footprint and 2.5meter height has 64 modules 66 and a total biofilm surface area of 2910square meters. The biofilm surface area is 315 square meters per cubicmeter of cassette volume and 786 square meters per square meter ofcassette footprint.

In comparison, a comparable sheet form module made with similar gastransfer membranes 14 can have a 1 mm thick fabric with a similar gastransfer surface area to biofilm surface area of 3.34. The sheets aremade into a module with horizontally apposed vertical headers. Themodule is 2 m long with 1.8 m of exposed membrane length. The sheets andmodule are 1 m high. The module is 0.3 m wide and can be operated in a1.5 m deep tank. The centre-to-centre spacing between adjacent sheets is8 mm. The biofilm surface area is 250 square meters per cubic meter ofcassette volume and 250 square meters per square meter of cassettefootprint.

As illustrated by the comparison above, the cord 10 module can have morebiofilm area per unit volume of module than a sheet form module.Further, tall sheet modules that rely on bubbles for liquid renewal orscouring have been known in the context of filtering membranes to beprone to a chimney effect whereby bubbles and liquid flow areconcentrate near the vertical midline of the sheets. This limits theheight of sheet modules. It is expected that a cord 10 module can behigher without a similar chimney effect which allows for an additionaldecrease in tank footprint and land consumption per unit biofilm area.

This written description uses examples to disclose the invention andalso to enable any person skilled in the art to practice the invention,including making and using any devices or systems and performing anyincorporated methods. The patentable scope of the invention is definedby the claims, and may include other examples that occur to thoseskilled in the art.

We claim:
 1. A cord comprising, a core extending along the length of thecord generally parallel with a longitudinal axis of the cord, the corecomprising a plurality of hollow fiber gas transfer membranes parallelwith the longitudinal axis of the cord; and, at least one spiralfilament wrapped around the core.
 2. The cord of claim 1 wherein thecore comprises one or more reinforcing filaments.
 3. The cord of claim 1wherein the at least one spiral filament wrapped around the corecomprises a multifilament reinforcing yarn.
 4. The cord of claim 1having an outside diameter of between 0.3 mm and 2.0 mm.
 5. The cord ofclaim 1 wherein the hollow fiber gas transfer membranes have outsidediameters of 200 microns or less.
 6. The cord of claim 1 wherein thehollow fiber gas transfer membranes have a wall thickness of 50 micronsor less.
 7. The cord of claim 1 wherein the sum of the circumferences ofthe hollow fiber gas transfer membranes is at least 1.5 times thecircumference of a circle having the outside diameter of the cord.
 8. Anapparatus comprising, a) a gas transfer membrane having first end and asecond end; b) a source of a gas in fluid communication with the firstend of the gas transfer membrane; and, c) an aerator in fluidcommunication with the second end of the gas transfer membrane.
 9. Theapparatus of claim 8 wherein the gas transfer membrane comprises ahollow fiber membrane.
 10. The apparatus of claim 8 wherein the gastransfer membrane is part of a cord according to claim
 1. 11. Theapparatus of claim 8 comprising a pressurized gas delivery systemconnected to the first end of the gas transfer membrane.
 12. Theapparatus of claim 8 wherein the gas transfer membrane is orientedvertically in a module and the aerator is located below the module. 13.The apparatus of claim 8 wherein the aerator comprises a transduceradapted to produce burst of bubbles from a continuous supply of gas. 14.A process comprising the steps of, a) immersing a gas transfer membranein water; b) flowing a gas into the gas transfer membrane; c)withdrawing an exhaust gas from the gas transfer membrane; and, d)producing bubbles comprising the exhaust gas in the water.
 15. Theprocess of claim 14 further comprising supporting a biofilm on the gastransfer membrane and transferring oxygen to the biofilm.
 16. Theprocess of claim 14 wherein the gas transfer membrane is orientedvertically and the bubbles are produced below or near the bottom of thegas transfer membrane.
 17. The process of claim 14 wherein the gastransfer membrane is a) a hollow fiber membrane or b) part of a cordaccording to claim
 1. 18. The process of claim 14 wherein the bubblesfurther comprise ambient air.
 19. The process of claim 14 wherein thebubbles are provided at a lower rate to renew liquid around the gastransfer membrane and at a higher rate to help control biofilmthickness.
 20. The process of claim 14 wherein the bubbles are providedintermittently or from a transducer adapted to produce burst of bubblesfrom a continuous supply of gas.